Systems involved in the active transport of metal ions subserve a diverse group of physiologic activities. One such transport system is the Ca2 ion pump in sarcoplasmic reticulum which plays an essential role in the regulation of molecular events underlying muscular contraction and relaxation. The duration of the active state in cardiac muscle is determined in part by the duration of the interval in which Ca2 ion is bound to myofibrillar regulatory proteins; consequently, the rate of removal of Ca2 ion by sarcoplasmic reticulum will influence the time course of muscle relaxation. Sarcoplasmic microsomes prepared from old rat hearts showed significantly less uptake than microsomes from young hearts over a range of physiologically relevant Ca2 ion concentrations. Estimates of the amount of protein in the preparation arising from other membranes revealed no significant age-related differences indicating that the difference in transport activity is not simply the result of a variable level of contamination. The age-dependent change in Ca2 ion transport activity is consistent with the observation that old rat hearts show a prolonged relaxation phase and suggests a possible biochemical mechanism for that change.